Conventionally, technical ideas for detecting lines of sight have been developed as computer input apparatuses designed for handicapped persons who cannot freely use their hands or their fingers. Also, detections of sight lines have been utilized as vehicle assisting means, while sight lines of car drivers are monitored. However, conventional sight line detecting apparatuses have been designed for mounting specific instruments on users, and these sight line detecting technical ideas constrain these users. As a result, there is such a problem that these conventional sight line detecting apparatuses are not so user-friendly. In order to solve this problem, very recently, technical ideas capable of detecting lines of sight (LOS) have been developed by using eyeball models.
In the case that the eyeball models are utilized, correct distances from camera positions up to face surfaces are calculated by simple methods in correct manners, resulting in one problem. For example, the apparatus described in Japanese Laid-open Patent Application No. Hei-8-297019 (vehicle-purpose sight line direction measuring apparatus) (will be referred to as “patent publication 1” hereinafter) corresponds to such an apparatus for detecting a sight line of a car driver, while a distance sensor is utilized as means for acquiring a distance from a position of a camera to a face surface of the driver. As the distance sensor, an ultrasonic type sensor, and the like are used. When the ultrasonic type sensor is employed, the following problems occur. That is, the entire sight line detecting apparatus becomes complex; the distance detecting precision is deteriorated if the simple sensor is employed; and if the detecting precision is increased, then the cost of the entire line detecting apparatus is increased.
Also, Information Processing Research Report 2001-HI-93, pages 47 to 54 (Sight Line Measuring System based upon Eyeball Shaped Model), and Eighth Image Sensing Symposium, pages 307 to 312, 2002 (Sight Line Measuring Method based upon Eyeball Model) (will be referred to as “non-patent publications 1 and 2” hereinafter) describe technical ideas capable of detecting directions as to lines of sight (LOS) by utilizing eyeball models. In the apparatuses of these publications, a distance from a camera (otherwise, point light source) up to a face surface must be measured in order that a near-infrared light is illuminated from the point light source to a cornea present in an eye so as to acquire coordinates of a reflection image (called as “Purkinje image”). As the distance measuring method, this distance is measured based upon a focusing value when a lens of the camera is focused on such a position that a dimension of the point light source (namely, dimension of Purkinje image) on an image photographed by the camera becomes minimum. However, it is practically difficult to adjust that the dimension of the Purkinje image correctly becomes minimum. As a consequence, the high-precision distance measurement cannot be expected, but also, the sight line direction by utilizing this measured distance contains errors, resulting in another problem. The Applicant of the present invention has already filed the invention capable of solving these problems (U.S. patent application Ser. No. 11/078,144, “LINE-OF-SIGHT DETECTING METHOD AND APPARATUS THEREFOR” filed on Mar. 11, 2005) (will be referred to as “patent publication 2” hereinafter).